Ms Jackson is also a former junior transport minister, and lost to Frank Dobson in Labour's last mayoral selection contest.

Ms Gavron, nicknamed "quango queen" after she was appointed to 16 Government bodies while a Haringey councillor, is the first Labour hopeful formally to declare her bid for the candidacy. Former sports minister Tony Banks and London Assembly chairman Trevor Phillips are expected to join the fray.

Others who will back Ms Gavron include MPs Karen Buck and John Cryer, and the Transport and General Workers' Union London region.

A close ally of Mr Livingstone, last week Ms Gavron supported the abortive effort to let him rejoin Labour and stand for re-election in 2004 under the party banner.

An expert on town planning and the brains behind this year's draft London Plan, she has said she wants Labour supporters to give the independent Mr Livingstone their second-preference votes.